Zinciferous metal-plated steel sheet has an excellent corrosion resistance and for this reason is widely used in applications such as, for example, building materials, household electrical appliances, and automobiles. Zinciferous metal plated steel sheet is frequently not painted when used as a structural member or as an interior component of household electrical appliances.
Zinciferous metal-plated steel sheet is ordinarily subjected to a chromate treatment in the case of such unpainted service in order to inhibit white rust. A surface-treatment process has also recently appeared that uses a resin containing chromate treatment bath. This process increases the added value by increasing the corrosion resistance, pressability, fingerprint resistance, and chromium fixation ratio of zinciferous metal-plated steel sheet.
However, when unpainted zinciferous metal-plated steel sheet that has been subjected to a conventional surface treatment as described above is used or stored in the atmosphere or in a very humid environment, its surface develops a gray/black appearance and its commercial value is strongly diminished as a result. This phenomenon is known as blackening. Blackening leads merely to a change in the appearance of zinciferous metal-plated steel sheet, but leaves its other properties almost unaltered. A vexing problem posed by blackening is that it tends to develop more readily as the corrosion resistance of the material increases.
Various processes for preventing blackening have already been proposed. For example, Japanese Patent Publication Number Hei 1-53353 [53,353/1989] teaches a process for treating the surface of Zn/Al alloy-plated steel sheet in order to prevent blackening. This process uses a treatment bath that contains chromic acid and/or chromate salt in combination with water soluble resin or emulsified resin. However, the results afforded by this process are still not completely satisfactory.
Japanese Patent Publication Number Hei 3-49982 [49,982/1991] also discloses a process for preventing blackening. In this process, hot-dip zinciferous metal-plated steel sheet is first treated with a bath that contains Co ion and/or Ni ion in order to deposit these metals on the surface. The surface of the sheet is then subjected to a chromate treatment. This process, however, consists of a two-step method (Ni and/or Co plating treatment and chromate treatment) or a three-step method (the preceding plus a water wash after the Ni and/or Co plating treatment) and is therefore difficult to implement in existing surface treatment facilities.